Administration of antibiotics to patients often causes diarrhea and in some cases this can progress to life-threatening pseudomembranous colitis. Evidence now supports the hypothesis that often the cause of this colitis is the elaboration of toxins in the colon by antibiotic-resistant clostridia. Clostridium difficile is most often suggested to be the major pathogen, but Clostridium sordellii also may be involved. I propose to isolate the toxins produced by these two species, characterize them in several ways, and prepare specific antitoxins. The antisera will be used to test stools from pseudomembranous colitis patients for the presence of these toxins. This research should provide a solid base for future work on the mechanism of action of these toxins and provide the specific antisera needed to determine how often these toxins are responsible for causing pseudomembranous colitis.